We formerly succeeded in developing water-resistant pearlescent pigment that can be applied to paint for outdoor use, particularly to paint for exterior coating of cars (Japanese Patent Application No. 276687/1986, equivalent to allowed U.S. application Ser. No. 07/123,474, filed Nov. 20, 1987).
Paint for outdoor use, for example, paint suitable for exterior coating of cars, must be such that the coated film shows no change in appearance when exposed to various weather conditions.
It is known that particles of titanium dioxide, if contained in a coated film, tend to induce oxidative decomposition of the polymer that constitutes the coated film by the action of near ultraviolet rays and moisture, thus leading to chalking. To suppress such an activity of titanium dioxide, many techniques have been proposed and adopted in commercial products, in which titanium dioxide is doped or coated with a chromium, silicon, aluminum, zinc, phosphorus or zirconium compound, either alone or in combination.
Aluminum flake pigment is also used in paint for exterior coating of cars, and it is known that paint of this type also tends to undergo changes in appearance when exposed to various weather conditions. Known methods for evaluating the tolerance of pigment to weather conditions, are outdoor exposure tests and accelerated weathering tests. When the water resistance alone is to be evaluated, coated boards are exposed to an atmosphere of high temperature and humidity (blister box test) or immersed in hot water (hot-water immersion test) to examine the resulting deterioration in gloss and changes in color.
It is generally accepted that such deterioration in gloss and changes in color are a result of light scattering caused by micropores in the coated film, which have been formed by permeation of water or steam to produce minute blisters, followed by evaporation of the water.
Pearlescent pigments comprising metal-oxide-coated mica (mica flakes with their surface coated with a metal oxide, such as titanium oxide, iron oxide and a combination thereof) have been widely employed in notions for daily use, toys, packaging materials and many other uses as colorants for paint, printing ink and plastics, and their application has recently been expanded to outdoor uses, such as exterior coating for cars and construction materials.
However, conventional pearlescent pigments, when submitted to the water resistance tests (blister box and hot-water immersion tests), show deterioration in gloss and changes in color as in the case with aluminum flake pigments, and hence it has been pointed out that some modification is required for pigments of this type to be applied to paint for outdoor use, particularly to paint for exterior coating of cars.
The pigment of the new type in U.S. Ser. No. 07/127,474 shows excellent gloss and water resistance without using any chromium compound, but is still insufficient in resistance to light. Application of this technique to mica flakes coated with metal oxides including titanium dioxide failed to fully suppress the adverse activity of titanium dioxide as mentioned above; coated film containing the thus-obtained pigment showed unsatisfactory results when subjected to tests involving light irradiation, such as outdoor exposure tests and accelerated weathering tests.
In the automotive industry, there has been an increasing demand for pearlescent gloss finish, and this requires the development of new pearlescent pigments for outdoor paint that contain no chromium compounds and show high gloss and outstanding weatherability.
This invention provides novel, weather-resistant pearlescent pigments that will meet the above-mentioned requirements, and processes for producing the same.